The present invention relates to a development apparatus for developing latent electrostatic images to visible or transferable images of high quality for use in electrophotographic copying apparatus, electrostatic recording apparatus, facsimile apparatus or other recording apparatus. More particularly, the present invention relates to a development apparatus of the above described type in which a development roller is in arc-contact with a latent-electrostatic-image-bearing flexible recording medium and is capable of supplying a sufficient amount of toner to the recording medium for development of the latent electrostatic images, without requiring any delicate adjustment of the gap between the development roller and the recording medium or of the gap between the development roller and a doctor blade for regulating the thickness of a toner layer on the development roller, and without increasing the rotary speed of the development roller to conventional high speeds, and in which the doctor blade is also capable of performing charge injection to the toner with high efficiency, whereby images can be developed with high quality with a minimum of deposition of toner on the background.
In a conventional development apparatus for developing latent electrostatic images by toner for use in an electrophotographic or electrostatic image recording system, the toner is supplied to a latent-electrostatic-image-bearing recording medium by a development roller carrying a thin layer of the toner thereon, regulated to a predetermined thickness by a doctor blade, with a certain gap maintained accurately between the development roller and the recording medium. Usually, in such a development apparatus, it is necessary to maintain accurately the gap between the development roller and the recording medium so as to be slightly greater than the gap between the development roller and the doctor blade. Therefore, it is necessary that the members for maintaining those gaps and other relevant members be made with high assembly accuracy.
Furthermore, in the above-described conventional apparatus, the effective development area between the development roller and the latent-electrostatic-image-bearing recording medium, where latent electrostatic images are developed by toner being transferred from the development roller to the recording medium, is inevitably small due to the above-described gap between the development roller and the recording medium and the conventional shapes of the development roller and the recording medium (the recording medium is typically a cylindrical drum). When the development area is small, there is the risk that a sufficient amount of toner for development will not always be supplied to the recording medium. Accordingly, there is the risk that images of high quality will not always be obtained. Therefore, in a conventional development apparatus of the above-described type, in order to minimize the above risk, the development roller is rotated at speeds as high as 4 to 5 times the peripheral rotary speed of the recording medium in an effort to guarantee availability of sufficient toner.
Furthermore, in the case where latent electrostatic images formed on a recording medium are developed to visible images by use of a high-resistivity one-component-type magnetic toner, it is necessary that the quantity of electric charges in each toner particle be great in order to obtain developed images of high quality and still not cause toner deposition on the background thereof.
In particular, in the case of an image-transfer type recording system employing an electrostatic recording process using a dielectric recording medium, it is extremely difficult to quench charges on the recording medium before formation of latent electrostatic images. Therefore, the recording medium is uniformly pre-charged to a certain surface potential before latent electrostatic images are formed, and latent electrostatic images are then formed by applying voltages to the pre-charged recording medium so as to partially quench the former charges in the form of latent electrostatic images, by use of a multi-stylus head or the like.
In the above-mentioned type recording system, if the quantity of charges in each toner particle is insufficient, the toner is deposited markedly on the background, and, as a matter of course, the obtained image quality is significantly reduced. In order to eliminate such shortcomings, in a conventional development apparatus of the type employing a development roller comprising a rotatable non-magnetic, metallic, cylindrical sleeve for forming a magnetic brush thereon, with a stationary inner magnetic roller therein, voltage is applied to the surface of a layer of magnetic toner on the non-magnetic sleeve so as to electrically charge the magnetic toner. However, by that method, the toner cannot be electrically charged sufficiently for obtaining high image quality with avoidance of deposition of the toner on the background.